1. Field of the Invention
The invention relates to a gas chromatograph and a multiport valve unit for a gas chromatograph.
2. Description of the Related Art
U.S. Pat. No. 6,453,725 B1 discloses a gas chromatograph with a multiport valve unit that serves sample dosing and switching between two chromatographic separating devices with downstream detectors. The multiport valve unit comprises ten controllable valves, which are connected in series and are open or closed depending on their activation. In each case, immediately adjacent valves of the series circuit are activated here in a different manner, so that the multiport valve unit has two different switching positions, in which each second valve is either closed or open.
At the start and end of the series circuit and at the different points of connection of the adjacent valves, a sample feed, a sample vent, an inlet and an outlet of a metering chamber, a first separating device with a subordinate first detector, a second separating device with a subordinate second detector and a gas outlet are connected such that, in a first switching position of the multiport valve unit, a sample taken from a technical process is conveyed in a continuous stream through the metering chamber, while at the same time the first separating device with the first detector are backflushed and parallel thereto the second separating device with the second detector are forward-flushed with a carrier gas. In a second switching position of the multiport valve unit, the sample quantity contained in the metering chamber is guided successively through the first and the second separating device via the carrier gas and here broken down here into different sample components, which are detected with the detectors. At the same time the sample stream is guided past the metering chamber.
US 2002/0131905 A1 discloses a modification of the previously described multiport valve unit which, between the first switching position, in which the sample is conveyed through the metering chamber, and the second switching position, in which the sample is conveyed out of the metering chamber into a separating device, adopts a further switching position, in which the metering chamber is briefly separated from the sample feed, in order to enable matching of the pressure in the metering chamber to the outside pressure.
The multiport valve unit of the conventional gas chromatograph has an upper part, a lower part and a central part in the form of a disk, which upon interpositioning of a first diaphragm is connected to the upper part and upon interpositioning of a second diaphragm is connected to the lower part. The upper part contains five recesses in its body surface abutting the first diaphragm, into which control air can be introduced via a first control line. The lower part contains five further recesses in its body surface abutting the second diaphragm, into which the control air can be introduced via a second control line. The central part contains in each case a pair of holes on its top abutting the first diaphragm and on its underside abutting the second diaphragm, in the areas opposite the ten recesses, where each pair of holes forms fluid ports in each case of one of the ten valves. Upon introduction of control air into the five recesses of the upper part, the first diaphragm closes the respectively opposite holes in the central part and the five valves are closed. At the same time, in the absence of control air in the further five recesses of the lower part, the second diaphragm recedes into the further five recesses as the holes opposite it are released, and the further five valves are open. The control air is introduced alternately into the five recesses of the upper part and the further five recesses of the lower part, so that the five valves and the further five valves open and close alternately. Formed in the central part are fluid channels, which connect holes on the top of the central part to holes on the underside and thus switch the five valves and the further five valves alternately in series. The fluid channels further lead to external ports, which are mounted on the central part, and serve to attach the different components of the gas chromatograph to the multiport valve unit.
Because of the dead volumes of the fluid channels in the central part of the known multiport valve unit, its use in certain applications may be limited. Thus, as already mentioned above, in the first switching position of the multiport valve unit, the sample taken from the technical process is guided through the metering chamber via one of the ten valves and in the second switching position transferred from the metering chamber into the chromatographic separating devices via a different valve via the carrier gas. At the point of switching, a part of the fluid channel between the valves involved forming the dead volume is filled with the sample, which then diffuses from the dead volume into the carrier gas, which leads to an imprecise injection of the sample into the carrier gas stream, connected with a reduction in the resolution of the subsequent chromatographic separation.
WO 2007/028130 A2 shows a very similar gas chromatograph that likewise has a multiport valve unit comprising ten controllable valves connected in series, which serves sample dosing and switching between two chromatographic separating devices. Here, immediately adjacent valves of the series circuit in each case are also differently activated, so that the multiport valve unit has two different switching positions, in which each second valve is either closed or open. In contrast to the conventional gas chromatograph shown in U.S. Pat. No. 6,453,725 B1, in the first switching position of the multiport valve unit both separating devices are backflushed with the carrier gas.
The multiport valve unit also differs in its structure from that known from U.S. Pat. No. 6,453,725 B1, as it has only one diaphragm and all ten recesses serving to introduce the control air are jointly formed in one component, which abuts one of the sides of the diaphragm with its body surface containing the recesses. Accordingly the pairs of holes that form the fluid ports of the individual valves are also formed together in a further component, which abuts the other side of the diaphragm with its side containing the pairs of holes. In the further component, fluid channels are formed in a V-shaped arrangement, which connect the respectively adjacent pairs of holes and thus switch the ten valves in series. The fluid channels further lead to external ports, which serve to attach the different components of the gas chromatograph to the multiport valve unit. Here, the limitations or problems explained above in relation to the injection of the sample into the carrier gas stream as a result of the dead volumes of the fluid channels also arise.
EP 0 400 016 B1 discloses a multiport valve unit with a lower part in the form of a disk, in which in each case a recess is formed in an outer surface for each valve, into which control air can be introduced via an individual control line. A diaphragm abuts the outer surface of the lower part with the recesses. The recesses can also be formed in an upper part or intermediate part in the form of a disk, which abuts the diaphragm on the opposite side. In the case of the intermediate part, this contains pairs of holes opening into the recesses, where each pair of holes forms fluid ports in each of the valves. On its side facing away from the diaphragm, the intermediate part contains grooves, which are covered by an overlying upper part and which connect the fluid ports connected in series to each other and/or lead from the fluid ports to prescribed locations, to which they are connected with external ports via drilled holes in the upper part.
In order to achieve precise sample dosing in a gas chromatograph, it is, for example, known from WO 2010/066571 A1 to convey the sample amount directed from the metering chamber by the carrier gas to an injector operating without valves, which diverts part of this sample amount and introduces it into the separating devices as a precisely delimited sample plug. A switching device likewise operating without valves is further provided between the first separating device with the downstream first detector and the second separating device with the downstream second detector, in order to convey sample components that are not sufficiently separated at the end of the first separating device into the second separating device, and keep sample components that are already sufficiently separated at the end of the first separating device and have been detected away from the second separating device. The controlling of the injector and the switching device occurs via pressure differences in gas paths, which necessitates precise pressure regulation and adjustment of the pressure drops via flow resistances. In addition, the switching device requires an auxiliary gas supply, which is connected to correspondingly high carrier gas consumption.